1. Field of the Invention
This invention relates to an inverter device, and more particularly to an inverter device for reducing the commutating time of the switching elements in a commutation type inverter.
2. Description of the Prior Art
The prior art commutation type inverter device is shown in FIG. 1. A D.C. power source 1 is connected through a pair of switching elements 2 and 3, such as power transistors, to a bridge type inverter circuit 4 including a plurality of thyristors which supplies A.C. power to a load 5. To feed back power from the load 5 to the D.C. power source a diode bridge 6 is connected in parallel with the D.C. Power source 1. To each base terminal of the transistors 2 and 3 signals from a well-known base signal generating device (not shown), are supplied.
The transistors 2 and 3 are in their OFF states only when the bridge type inverter circuit is caused to commutate; otherwise the transistors 2 and 3 are in their ON states.
The operation of the prior art device shown in FIG. 1 is herein described with reference to the timing diagram shown in FIG. 2.
At time t.sub.0, the transistors 2 and 3 are in their ON states and the thyristors S.sub.3, S.sub.5 and S.sub.4 of the bridge type inverter circuit 4 also are in their ON states.
At time t.sub.1, to cause the thyristor S.sub.3 to go into its OFF state the transistor 2 is caused to go into its OFF state. After the forward blocking voltage of the thyristor S.sub.3 is recovered, both the thyristor S.sub.6 and transistor 2 are caused to go into their ON states. The commutating operation is achieved by the above-mentioned steps.
The prior inverter device, however, has a defect in that it requires an extremely long time i.e. turn-off time of the thyristor, for recovery of the forward blocking voltage of the thyristor because a reverse voltage is not applied to the thyristor and the moving time of the stored charge in the thyristor is long even if either transistor 2 or 3 is caused to go into its OFF state.
It is, therefore, not favorable to use a relatively high frequency type inverter since the bridge type inverter circuit cannot commutate at a high frequency, i.e. with a short period.